Digital oscilloscopes generally use raster scan displays to present the activity of electrical signals to their users. Each raster scan display, such as those seen every day on computer screens, consists of a two dimensional array of pixels, with each pixel location being uniquely defined by a row number and column number. The simplest and lowest cost versions of such displays are "single bit" displays, in that the memory from which they derive the information to be displayed only has one bit of intensity information associated with each pixel. In such a display that single bit of information determines whether the pixel associated with it is either "on" or "off", with "on" dictating that a predetermined amount of intensity is to be used to illuminate the pixel and "off" indicating that the pixel is not to be illuminated at all.
The more complex and expensive alternative to a single bit display is a multi-bit display, which can provide variable intensity (also known as "gray-scale") or color variations as a substitute indicator of brightness. The memory locations associated with each pixel of a variable intensity display contain multiple bits of intensity information, indicating the number of varying intensity levels with which they can be illuminated. Like the pixels of single bit displays, those of multi-bit displays can exhibit an "off" or dark state, but instead of one value of illumination, they have multiple values. Typically, the number of values available is 2.sup.N -1, where N is the memory depth at each address of the raster memory. Thus, for example, a four bit deep raster scan memory can support fifteen levels of partial through maximum illumination, as well as the dark or "off" state. Pixel intensity can also be translated into differing colors, as well as intensity or "brightness".
With this larger amount of data, multi-bit displays can convey more information about the behavior of electrical signal waveforms under observation, particularly if the signal is not perfectly repetitive and therefore has less activity in some portions than others. U.S. Pat. No. 4,940,931 to Katayama et al. for "Digital Waveform Measuring Apparatus Having A Shading-tone Display", hereby incorporated by reference, describes a system for producing digital variable intensity displays.
Typically, digital oscilloscopes acquire information about the behavior of a circuit node by periodically sampling the voltage present at the node. The oscilloscope probe tip is placed in contact with the node and the probe and front end of the oscilloscope precisely replicate the signal, or some predetermined fraction or multiple of the signal, and present it to an analog-to-digital converter. The output of the analog-to-digital converter is a series of multi-bit digital words that are stored in an acquisition memory. Successively acquired samples are stored at sequentially related addresses in the acquisition memory, and are thereby related to a time scale. Those addresses will eventually be converted back to a time scale, one of which is represented as horizontal distance along the x-axis of the oscilloscope's raster scan display.
In a typical digital oscilloscope, voltage amplitude values derived from the data contents of an acquisition memory location determine the vertical location (row number) of an illuminated pixel, while time values derived from the addresses of the acquisition memory determine the horizontal location (column number). The process of expanding the contents and addresses of an acquisition memory to produce contents for a two dimensional raster memory is known as "rasterization".
Multi-bit intensity information also makes it possible to create analog-like "persistence" effects, i.e., the decay of signal intensity over time. In the older analog oscilloscopes, persistence was a decay of the illumination of the cathode ray tube (CRT) that was a function of the type of phosphor used in the construction of the CRT and the voltages applied to different elements of that tube. In digital oscilloscopes, a persistence decay function can be implemented by decrementing the intensity value associated with each illuminated pixel according to some algorithm. U.S. Pat. No. 4,504,827 to Hanson et al. for "Synthetic Persistence for Raster Scan Displays", hereby incorporated by reference, describes a method for pseudo randomly decrementing intensity data in a raster scan display. U.S. Pat. No. 5,254,983 to Long et al. for "Digitally Synthesized Gray Scale for Raster Scan Oscilloscope Display", hereby incorporated by reference, describes one approach for persistence-like decay of acquired waveforms stored as digital numbers. U.S. Pat. No. 5,387,896 to Alappat et al. for "Rasterscan Display with Adaptive Decay", hereby incorporated by reference, describes a system for rasterization that operates on a local pixel in one of two ways, depending on a calculation based on that pixel's initial value.
For many users, especially those having some experience with analog oscilloscopes, variable brightness usefully communicates information about the activity of the signal being observed. Many of these users have had a strong preference for some behaviors that resemble those of analog oscilloscopes. For example, as an analog oscilloscope generates vertical excursions during a horizontal sweep interval to provide a real-time picture of the signal activity at the probe tip, they inherently tend to vary the brightness of the display as an inverse function of the slope of the line they produce. This occurs because the cathode electron gun of the CRT generates a constant supply of electrons that depends on the setting of a "brightness" control, and the length of the trajectory covered in a unit of time is minimally determined by the x-axis distance associated with any particular sweep speed, but is increased by any and all y-axis excursions. And a y-axis excursion can be a large multiple of the corresponding x-axis distance, so the constant available electron beam energy appears to be reduced by a large factor as it is spread over this much longer distance. Thus, analog oscilloscopes inherently vary the brightness of the line they draw as an inverse function of the slope of that line.
Another even more highly desired feature of an analog oscilloscope or a digital oscilloscope with a high waveform throughput, is the ability to detect an intermittent signal anomaly that occurs in an otherwise repetitive signal. Older digital oscilloscopes, with low "live time" make observing intermittent signal activity improbable, at least in the absence of special trigger modes designed to detect certain classes of intermittent signal activity. Analog oscilloscopes will show a faint trace indicating the presence of this intermittently anomalous signal behavior. Of course, if the signal becomes too intermittent, the trace will be so faint in brightness that it may be missed entirely by the oscilloscope operator.
With the persistence decay feature turned off, i.e., infinite persistence, a digital oscilloscope with single bit (on/off) intensity information will display rare or unusual waveforms with the same intensity as highly repetitive ones, i.e., "on". Digital oscilloscopes with multi-bit raster memories, and that therefore can provide variable intensity (or variable color) displays, allow for a visual distinction to be made between rare and repetitive waveforms. However, unless the persistence feature is turned off, these oscilloscopes may not illuminate truly rare events with enough intensity for a long enough period of time to allow the operator to notice, much less analyze, the intermittent activity.
Current oscilloscope products from assignee corporation, Tektronix, provide a means whereby the operator can distinguish between the most recent individual waveform acquisitions and older waveforms acquired previously. Both the TDS300 and TDS200 oscilloscopes use "off" and two levels of "on", with each level of "on" having different intensity levels. The most recently drawn waveforms are shown in the full level of "on"intensity, which is bright for the TDS300 series and black for TDS200 series oscilloscopes.
The older "historical" waveforms are shown in lower, secondary intensity level (dim for TDS300, gray for TDS200). The historical information stays at-that secondary intensity level for the entire duration of an acquisition series or for a preselected persistence time. (The term "acquisition series" as used in this document refers to a sequence of individual waveform acquisitions taken at the same settings over time in response to a series of separate triggers. A single or individual "acquisition" refers to one waveform record taken in response to a single trigger.
A simplified form of persistence mode has been included even in these relatively low-cost digital oscilloscopes to facilitate the collecting of signal history on a single screen. Even a simplified version can be quite helpful in locating glitches or other rare events. The TDS300 uses a simple collect and erase form of persistence, all done in one display plane. The user sets the desired persistence time, p, in seconds, and the oscilloscope collects the rasterized results of individual waveform acquisitions onto the same display plane for that amount of time. As the time p passes, the latest acquisition is displayed at full intensity, while all others are displayed at the other single level of reduced intensity. After the persistence interval p expires, the entire display plane is cleared and the same process is repeated. The disadvantage in this approach is that at any particular time, t, within the persistence interval p, the currently viewable history is only a maximum of the user's selected time interval, i.e., t modulo p. Information collected near the end of the interval disappears before it can be analyzed, and just after the display screen has been cleared no history information is visible at all.
The TDS200 uses a somewhat superior, multi-plane collect and erase model. Instead of collecting individual waveform acquisitions into a single display plane and then erasing it and using it again, it collects individual waveform acquisitions into one of N separate display planes for a selected time p. It ORs all N display planes to produce the display. Every p/N seconds the oldest display plane is cleared, and the next set of acquisitions are then collected into the newly cleared plane. This approach is superior, because at any given time, t, a history of (p--p/N+(t mod p/N)) seconds can be seen by the user. The most recent plane is displayed at the maximum level of intensity, while the other ORed planes are displayed at the single other reduced level of intensity.
What is desired is a method of reacting to the acquisition and detection of unusual waveforms that makes it easy for an operator to notice and analyze or record them.